Because vehicular tires support the sprung mass of a vehicle on a road surface and such tires are resilient, any irregularities in the dimensions of the tire or in its resiliency, any dimensional irregularities in the wheel rim, and/or any dynamic imbalance of the wheel rim and tire assembly will cause undue vibrations to be transmitted to the sprung mass of the vehicle thereby producing undesirable ride characteristics commonly known as "smooth road shake". One technique almost always used to reduce smooth road shake is to balance the wheel rim and tire assembly. For a number of years, the industry attempted to further reduce smooth road shake by measuring the unloaded radial and lateral runout of tires mounted on a wheel rim and removed portions of the tire tread to reduce the unloaded radial and lateral runout with a tire truing machine. Such machines are emplified in U.S. Pat. Nos. 3,752,207; 2,966,011; and 2,918,116. Because smooth road shake is generated when the tire is loaded by vehicular weight, the tire truing machines have not been successful in eliminating smooth road shake even when balancing was used. More recently, the automotive industry has begun using a technique known as "force variation tire grinding" in addition to balancing in an attempt to reduce the undue vibrations transmitted in the sprung mass of the vehicle. A number of devices using this technique have been proposed such as those shown by U.S. pat. Nos. 3,553,903; 3,681,877; and 3,725,137. While such devices have been relatively successful in reducing the undue vibrations transmitted to the sprung mass of the vehicle by the tires, their complexity, manufacturing cost, and the requirement of trained operating personnel has limited the use of these device primarily to the manufacturing facilities of the vehicle tire manufacturing companies. This has resulted in improved ride characteristics with respect to the original equipment tires on the vehicle but has done little to maintain the original improved ride characteristics when these original equipment tires are replaced by the second market. Further, these prior art devices have usually mounted the tire on an axle or arbor for testing rather than on the vehicular wheel rim. Because the wheel rim itself can have dimensional inaccuracies which affect the undue vibrations transmitted to the sprung mass of the vehicle, correcting the tires with force variation tire grinding without the tire being mounted on the wheel rim on which it is to be used with the vehicle frequently failed to compensate for the total irregularities of the wheel rim and tire combination even in combination with balancing.
In an attempt to compensate for the undue vibrations transmitted to the sprung mass of the vehicle by the wheel rim and tire combination in addition to balancing, a machine has been suggested that reduces the loaded radial runout of the wheel rim and tire combination while the wheel rim and tire combination is mounted on the vehicle. Such a device is disclosed in R. J. Caulfield and R. J. Higgins: "On-Car Tire Grinder for Improved Ride Smoothness" presented at the National Automobile Engineering Meeting, Detroit, Mich. (May, 1972) SAE Paper No. 720465. A similar device is shown in U.S. Pat. No. 3,905,160. Because these on-the-car grinders use the weight of the vehicle to load the wheel rim and tire combination, a great deal of effort must be expended to insure that the vehicle is substantially level and to insure that the sprung mass of the vehicle is properly supported by the wheel rim and tire combination in order to obtain proper loading during testing and grinding. Because the loading drum is also used to rotate the wheel rim and tire combination and the spacial relationship of the grinding rasps which grind the tire is controlled from the support frame carrying the drum, any slippage between the loading drum and tire results in the wheel rim and tire combination being improperly corrected. Further, because the rotational axis of the loading drum and the pivot axis of the grinding rasps support arm are held stationary in the grinder frame while the rotational axis of the wheel rim and tire combination can move with respect to both the drum rotational axis and the grinding arm pivot axis, the angular spacing between the loading point on the tire and the grinding point on the tire varies as the wheel rim and tire combination moves with respect to the loading drum rotational axis. This also causes improperly corrected tires.